Hot-Hole versus Hot-Electron Transport at Cu/GaN Heterojunction Interfaces

Revealing the Loss Mechanism of Chemically-Induced Hot Electron Transport

Hot electrons are crucial for unraveling the intrinsic relationship between chemical reactions and charge transfer in heterogeneous catalysis. Significant research focused on real-time detection of reaction-driven hot electron flow (chemicurrent) to elucidate the energy conversion mechanisms, but it remains elusive because carrier generation contributes to only part of the entire process. Here, a theoretical model for quantifying the chemicurrent yield is presented by clarifying the contributions of hot carrier losses from the internal emission and multiple reflections. The experimental chemicurrent yield verifies our model with a reliable mean free path of hot electrons, emphasizing the importance of comprehensive consideration of the transport process besides hot electron generation. Moreover, Pt nanoparticles (NPs)-decorated Au/TiO2 is examined, showing the role of NPs-induced carrier losses in the performance of catalytic nanodiodes. These findings are expected to contribute to understanding the hot electron detection efficiency and designing nanodiodes with enhanced hot carrier flow and catalytic activity.

Surface-enhanced photoluminescence and Raman spectroscopy of single molecule confined in coupled Au bowtie nanoantenna

Optical nanoantennas possess broad applications in the fields of photodetection, environmental science, biosensing and nonlinear optics, owing to their remarkable ability to enhance and confine the optical field at the nanoscale. In this article, we present a theoretical investigation of surface-enhanced photoluminescence spectroscopy for single molecules confined within novel Au bowtie nanoantenna, covering a wavelength range from the visible to near-infrared spectral regions. We employ the finite element method to quantitatively study the optical enhancement properties of the plasmonic field, quantum yield, Raman scattering and fluorescence. Additionally, we systematically examine the contribution of nonlocal dielectric response in the gap mode to the quantum yield, aiming to gain a better understanding of the fluorescence enhancement mechanism. Our results demonstrate that altering the configuration of the nanoantenna has a significant impact on plasmonic sensitivity. The nonlocal dielectric response plays a crucial role in reducing the quantum yield and corresponding fluorescence intensity when the gap distance is less than 3 nm. However, a substantial excitation field can effectively overcome fluorescence quenching and enhance the fluorescence intensity. By optimizing nanoantenna configuration, the maximum enhancement of surface-enhanced Raman can be turned to 9 and 10 magnitude orders in the visible and near-infrared regions, and 3 and 4 magnitude orders for fluorescence enhancement, respectively. The maximum spatial resolutions of 0.8 nm and 1.5 nm for Raman and fluorescence are also achieved, respectively. Our calculated results not only provide theoretical guidance for the design and application of new nanoantennas, but also contribute to expanding the range of surface-enhanced Raman and fluorescence technology from the visible to the near-infrared region.

Transport and Interfacial Injection of d-Band Hot Holes Control Plasmonic Chemistry

Harnessing nonequilibrium hot carriers from plasmonic metal nanostructures constitutes a vibrant research field with the potential to control photochemical reactions, particularly for solar fuel generation. However, a comprehensive understanding of the interplay of plasmonic hot-carrier-driven processes in metal/semiconducting heterostructures has remained elusive. In this work, we reveal the complex interdependence among plasmon excitation, hot-carrier generation, transport, and interfacial collection in plasmonic photocatalytic devices, uniquely determining the charge injection efficiency at the solid/liquid interface. Measuring the internal quantum efficiency of ultrathin (14-33 nm) single-crystalline plasmonic gold (Au) nanoantenna arrays on titanium dioxide substrates, we find that the performance of the device is limited by hot hole collection at the metal/electrolyte interface. Our solid- and liquid-state experimental approach, combined with ab initio simulations, demonstrates more efficient collection of high-energy d-band holes traveling in the [111] orientation, enhancing oxidation reactions on {111} surfaces. These findings establish new guidelines for optimizing plasmonic photocatalytic systems and optoelectronic devices.

Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots

Metal-enhanced photoluminescence is able to provide a robust signal even from a single emitter and is promising in applications in biosensors and optoelectronic devices. However, its realization with semiconductor nanocrystals (e.g., quantum dots, QDs) is not always straightforward due to the hidden and not fully described interactions between plasmonic nanoparticles and an emitter. Here, we demonstrate nonclassical enhancement (i.e., not a conventional electromagnetic mechanism) of the QD photoluminescence at nonplasmonic conditions and correlate it with the charge exchange processes in the system, particularly with high efficiency of the hot-hole generation in gold nanoparticles and the possibility of their transfer to QDs. The hole injection returns a QD from a charged nonemitting state caused by hole trapping by surface and/or interfacial traps into an uncharged emitting state, which leads to an increased photoluminescence intensity. These results open new insights into metal-enhanced photoluminescence, showing the importance of the QD surface states in this process.

Active Site Engineering on Plasmonic Nanostructures for Efficient Photocatalysis

Plasmonic nanostructures have shown immense potential in photocatalysis because of their distinct photochemical properties associated with tunable photoresponses and strong light-matter interactions. The introduction of highly active sites is essential to fully exploit the potential of plasmonic nanostructures in photocatalysis, considering the inferior intrinsic activities of typical plasmonic metals. This review focuses on active site-engineered plasmonic nanostructures with enhanced photocatalytic performance, wherein the active sites are classified into four types (i.e., metallic sites, defect sites, ligand-grafted sites, and interface sites). The synergy between active sites and plasmonic nanostructures in photocatalysis is discussed in detail after briefly introducing the material synthesis and characterization methods. Active sites can promote the coupling of solar energy harvested by plasmonic metal to catalytic reactions in the form of local electromagnetic fields, hot carriers, and photothermal heating. Moreover, efficient energy coupling potentially regulates the reaction pathway by facilitating the excited state formation of reactants, changing the status of active sites, and creating additional active sites using photoexcited plasmonic metals. Afterward, the application of active site-engineered plasmonic nanostructures in emerging photocatalytic reactions is summarized. Finally, a summary and perspective of the existing challenges and future opportunities are presented. This review aims to deliver some insights into plasmonic photocatalysis from the perspective of active sites, expediting the discovery of high-performance plasmonic photocatalysts.

Understanding Wavelength-Dependent Synergies between Morphology and Photonic Design in TiO2-Based Solar Powered Redox Cells

Solar powered redox cells (SPRCs) are promising for large-scale and long-term storage of solar-energy, particularly when coupled with redox flow batteries (RFBs). While efforts have primarily focused on heterostructure engineering, the potential of synergistic morphology and photonic design has not been carefully studied. Here, we investigate the wavelength-dependent effects of light-absorption and charge transfer characteristics on the performance of gold decorated TiO₂-based SPRC photoanodes operating with RFB-compatible redox couples. Through an in-depth optical and photoelectrochemical characterization of three complementary TiO₂ microstructures, namely nanotubes, honeycombs, and nanoparticles, we elucidate the combined effects of nanometer-scale semiconductor morphology and plasmonic design across the visible spectrum. In particular, thin-walled TiO₂ nanotubes exhibit a ∼ 50% increase in solar-to-chemical efficiency (STC) compared to thick-walled TiO₂ honeycombs thanks to improved charge transfer. Au nanoparticles both increase generation and interfacial charge transfer (above bandgap) and promote hot carrier injection (below bandgap) leading to a further 25% increase in STC. Overall, Au/TiO₂ nanotubes achieve a high photocurrent at 0.098 mA/cm² and an excellent STC of 0.06%, among the highest with respect to the theoretical limit. The incident photon to current efficiency and internal quantum efficiency are also superior to those of bare TiO₂ showing maximum values of 54.7% and 67%, respectively. Overall, nanophotonic engineering that synergistically combines morphology optimization and plasmonic sensitization schemes offer new avenues for improving rechargeable solar-energy technologies such as solar redox flow batteries.

Determining the role of Pd catalyst morphology and deposition criteria over large area plasmonic metasurfaces during light-enhanced electrochemical oxidation of formic acid

The use of metal composites based on plasmonic nanostructures partnered with catalytic counterparts has recently emerged as a promising approach in the field of plasmon-enhanced electrocatalysis. Here, we report on the role of the surface morphology, size, and anchored site of Pd catalysts coupled to plasmonic metasurfaces formed by periodic arrays of multimetallic Ni/Au nanopillars for formic acid electro-oxidation reaction (FAOR). We compare the activity of two kinds of metasurfaces differing in the positioning of the catalytic Pd nanoparticles. In the first case, the Pd nanoparticles have a polyhedron crystal morphology with exposed (200) facets and were deposited over the Ni/Au metasurfaces in a site-selective fashion by limiting their growth at the electromagnetic hot spots (Ni/Au-Pd@W). In contrast, the second case consists of spherical Pd nanoparticles grown in solution, which are homogeneously deposited onto the Ni/Au metasurface (Ni/Au-Pd@M). Ni/Au-Pd@W catalytic metasurfaces demonstrated higher light-enhanced FAOR activity (61%) in comparison to the Ni/Au-Pd@M sample (42%) for the direct dehydrogenation pathway. Moreover, the site-selective Pd deposition promotes the growth of nanoparticles favoring a more selective catalytic behavior and a lower degree of CO poisoning on Pd surface. The use of cyclic voltammetry, energy-resolved incident photon to current conversion efficiency, open circuit potential, and electrochemical impedance spectroscopy highlights the role of plasmonic near fields and hot holes in driving the catalytic enhancement under light conditions.

Tunable Nanoplasmonic Photodetectors

Visible and infrared photons can be detected with a broadband response via the internal photoeffect. By use of plasmonic nanostructures, i.e., nanoantennas, wavelength selectivity can be introduced to such detectors through geometry-dependent resonances. Also, additional functionality, like electronic responsivity switching and polarization detection, has been realized. However, previous devices consisted of large arrays of nanostructures to achieve detectable photocurrents. Here we show that this concept can be scaled down to a single antenna level, resulting in detector dimensions well below the resonance wavelength of the device. Our design consists of a single electrically connected plasmonic nanoantenna covered with a wide-bandgap semiconductor allowing broadband photodetection in the visible/near-infrared via injection of hot carriers. We demonstrate electrical switching of the color sensitivity as well as polarization detection. Our results hold promise for the realization of ultrasmall photodetectors with advanced functionality.

Flexible Plasmonics Using Aluminum and Copper Epitaxial Films on Mica

We demonstrate here the growth of aluminum (Al), copper (Cu), gold (Au), and silver (Ag) epitaxial films on two-dimensional, layered muscovite mica (Mica) substrates via van der Waals (vdW) heteroepitaxy with controllable film thicknesses from a few to hundreds of nanometers. In this approach, the mica thin sheet acts as a flexible and transparent substrate for vdW heteroepitaxy, which allows for large-area formation of atomically smooth, single-crystalline, and ultrathin plasmonic metals without the issue of film dewetting. The high-quality plasmonic metal films grown on mica enable us to design and fabricate well-controlled Al and Cu plasmonic nanostructures with tunable surface plasmon resonances ranging from visible to the near-infrared spectral region. Using these films, two kinds of plasmonic device applications are reported, including (1) plasmonic sensors with high effective index sensitivities based on surface plasmon interferometers fabricated on the Al/Mica film and (2) Cu/Mica nanoslit arrays for plasmonic color filters in the visible and near-infrared regions. Furthermore, we show that the responses of plasmonic nanostructures fabricated on the Mica substrates remain unaltered under large substrate bending conditions. Therefore, the metal-on-mica vdW heteroepitaxy platform is suitable for flexible plasmonics based on their bendable properties.

Progress of GaN-Based Optoelectronic Devices Integrated with Optical Resonances

Being direct wide bandgap, III-nitride (III-N) semiconductors have many applications in optoelectronics, including light-emitting diodes, lasers, detectors, photocatalysis, etc. Incorporation of III-N semiconductors with high-efficiency optical resonances including surface plasmons, distributed Bragg reflectors and micro cavities, has attracted considerable interests for upgrading their performance, which can not only reveal the new coupling mechanisms between optical resonances and quasiparticles, but also unveil the shield of novel optoelectronic devices with superior performances. In this review, the content covers the recent progress of GaN-based optoelectronic devices integrated with plasmonics and/or micro resonators, including the LEDs, photodetectors, solar cells, and light photocatalysis. The authors aim to provide an inspiring insight of recent remarkable progress and breakthroughs, as well as a promising prospect for the future highly-integrated, high speed, and efficient GaN-based optoelectronic devices.

Enhanced Performances of n-ZnO Nanowires/p-Si Heterojunctioned Pyroelectric Near-Infrared Photodetectors via the Plasmonic Effect

Although pyroelectric photodetectors have been intensively studied, the transient temperature change rate of pyroelectric materials is a main restrictive factor for improving the performance. In this work, we fabricate an ultrafast response self-powered near-infrared (NIR) photodetector (PD) based on Au nanoparticles (NPs) coated an n-ZnO nanowires (NWs)/p-Si heterojunction. The local surface plasmon resonance (LSPR) effect generated at the local contacts of Au NPs/ZnO NWs can significantly enhance the transient temperature change rate of the ZnO material to improve the photoresponse performances of the NIR PD. Compared with that in the pristine ZnO-based PD, the response time of the Au-coated NIR PD is decreased from 113 to 50 μs at the rising edge and 200 to 70 μs at the falling edge. Optical responsivity and detectivity of the Au-coated ZnO-based PD are increased by 212 and 266%, respectively. The pyroelectric current gain is produced by injecting hot electrons from the LSPR effect of Au NPs into the ZnO material and the thermal energy transfer caused by the photothermal effect of plasmonic Au nanostructure. This work provides an in-depth understanding of plasmonic effect-enhanced pyroelectric effect and presents a unique strategy for developing high-performance NIR photodetectors.

Anisotropic Ballistic Transport Revealed by Molecular Nanoprobe Experiments

Atomic-scale charge transport properties are not only of significant fundamental interest but also highly relevant for numerous technical applications. However, experimental methods that are capable of detecting charge transport at the relevant single-digit nanometer length scale are scarce. Here we report on molecular nanoprobe experiments on Pd(110), where we use the charge carrier-driven switching of a single cis-2-butene molecule to detect ballistic transport properties over length scales of a few nanometers. Our data demonstrate a striking angular dependence with a dip in the charge transport along the [11[over ¯]0]-oriented atomic rows and a peak in the transverse [001] direction. The narrow angular width of both features and distance-dependent measurements suggest that the nanometer-scale ballistic transport properties of metallic surfaces are significantly influenced by the atomic structure.

Challenges in Plasmonic Catalysis

The use of nanoplasmonics to control light and heat close to the thermodynamic limit enables exciting opportunities in the field of plasmonic catalysis. The decay of plasmonic excitations creates highly nonequilibrium distributions of hot carriers that can initiate or catalyze reactions through both thermal and nonthermal pathways. In this Perspective, we present the current understanding in the field of plasmonic catalysis, capturing vibrant debates in the literature, and discuss future avenues of exploration to overcome critical bottlenecks. Our Perspective spans first-principles theory and computation of correlated and far-from-equilibrium light-matter interactions, synthesis of new nanoplasmonic hybrids, and new steady-state and ultrafast spectroscopic probes of interactions in plasmonic catalysis, recognizing the key contributions of each discipline in realizing the promise of plasmonic catalysis. We conclude with our vision for fundamental and technological advances in the field of plasmon-driven chemical reactions in the coming years.

Direct Observation of Photoinduced Charge Separation at Transition-Metal Nitride-Semiconductor Interfaces

Optically excited hot carriers from metallic nanostructures forming metal-semiconductor heterostructures are advantageous for enhancing photoelectric conversion in the sub-band gap photon energy regime. Plasmonic gold has been widely used for hot carrier excitation, but recent works have demonstrated that plasmonic transition-metal nitrides have higher efficiencies in injecting hot electrons to adjacent n-type semiconductors and are more cost-effective. To collect direct evidence of hot carrier excitation from nanostructures, imaging of hot carriers is essential. In this work, photoexcited Kelvin probe force microscopy (KPFM) is used to image hot carriers excited in transition-metal nitride nanostructures forming heterostructures with semiconductors. Among available transition-metal nitrides, we select zirconium nitride (ZrN) for this study. Additionally, both p-type and n-type titanium dioxides (TiO₂) are selected to study the transport of hot holes and hot electrons. The KPFM results indicate that for ZrN and p-type TiO₂ heterostructures, hot holes are injected into the p-type TiO₂ across the Schottky contact. In the case of ZrN and n-type TiO₂ heterostructures, hot electrons are injected into the n-type TiO₂ across the ohmic contact. Because transition-metal nitrides are known to be more effective than gold at injecting hot carriers into adjacent semiconductors, unambiguously determining the mechanisms of hot carrier transportation of transition-metal nitrides using photoexcited KPFM will facilitate additional studies on hot carrier applications with transition-metal nitrides.

In Situ Visualization of Localized Surface Plasmon Resonance-Driven Hot Hole Flux

Nonradiative surface plasmon decay produces highly energetic electron-hole pairs with desirable characteristics, but the measurement and harvesting of nonequilibrium hot holes remain challenging due to ultrashort lifetime and diffusion length. Here, the direct observation of LSPR-driven hot holes created in a Au nanoprism/p-GaN platform using photoconductive atomic force microscopy (pc-AFM) is demonstrated. Significant enhancement of photocurrent in the plasmonic platforms under light irradiation is revealed, providing direct evidence of plasmonic hot hole generation. Experimental and numerical analysis verify that a confined |E|-field surrounding a single Au nanoprism spurs resonant coupling between localized surface plasmon resonance (LSPR) and surface charges, thus boosting hot hole generation. Furthermore, geometrical and size dependence on the extraction of LSPR-driven hot holes suggests an optimized pathway for their efficient utilization. The direct visualization of hot hole flow at the nanoscale provides significant opportunities for harnessing the underlying nature and potential of plasmonic hot holes.